Light-emitting thyristor, light-emitting element chip, optical print head, and image forming device

ABSTRACT

A light-emitting thyristor includes a first semiconductor layer of a P type, a second semiconductor layer of an N type arranged adjacent to the first semiconductor layer; a third semiconductor layer of the P type arranged adjacent to the second semiconductor layer; and a fourth semiconductor layer of the N type arranged adjacent to the third semiconductor layer. A part of the first semiconductor layer is an active layer adjacent to the second semiconductor layer. A dopant concentration of the active layer is higher than or equal to a dopant concentration of the third semiconductor layer. A thickness of the third semiconductor layer is thinner than a thickness of the second semiconductor layer. A dopant concentration of the second semiconductor layer is lower than the dopant concentration of the third semiconductor layer.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a light-emitting thyristor, alight-emitting element chip including the light-emitting thyristor, anoptical print head including the light-emitting element chip, and animage forming device including the optical print head.

2. Description of the Related Art

Conventionally, image forming devices of the electrophotographic type,equipped with an optical print head including a plurality oflight-emitting elements as an exposure device, have been widespread. Insuch an image forming device, an electrostatic latent image is formed onthe surface of a photosensitive drum by applying light emitted from theoptical print head to the surface of the photosensitive drum. As thelight-emitting elements included in the optical print head,light-emitting thyristors as three-terminal light-emitting elements havebeen well known (see Japanese Patent Application Publication No.2010-239084, for example).

However, a more excellent light emission property is being required inthe conventional light-emitting thyristors.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a light-emittingthyristor having an excellent light emission property, a light-emittingelement chip including the light-emitting thyristor, an optical printhead including the light-emitting element chip, and an image formingdevice including the optical print head.

A light-emitting thyristor according to an aspect of the presentinvention includes: a first semiconductor layer of a first conductivitytype; a second semiconductor layer of a second conductivity type, thesecond semiconductor layer being arranged adjacent to the firstsemiconductor layer; a third semiconductor layer of the firstconductivity type, the third semiconductor layer being arranged adjacentto the second semiconductor layer; and a fourth semiconductor layer ofthe second conductivity type, the fourth semiconductor layer beingarranged adjacent to the third semiconductor layer. A part of the firstsemiconductor layer is an active layer adjacent to the secondsemiconductor layer. A dopant concentration of the active layer ishigher than or equal to a dopant concentration of the thirdsemiconductor layer. A thickness of the third semiconductor layer isthinner than a thickness of the second semiconductor layer. A dopantconcentration of the second semiconductor layer is lower than the dopantconcentration of the third semiconductor layer.

According to the present invention, a light-emitting thyristor and alight-emitting element chip which have an excellent light emissionproperty can be provided. Further, according to the present invention,an optical print head and an image forming device which can improve thequality of a print image can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings,

FIG. 1 is a schematic plan view showing the structure of alight-emitting thyristor according to a first embodiment of the presentinvention;

FIG. 2 is a schematic cross-sectional view showing the structure of thelight-emitting thyristor according to the first embodiment, namely, thecross-sectional structure at the line A-B-C in FIG. 1;

FIGS. 3A to 3C are cross-sectional views showing a fabrication processusing an etching stop layer;

FIGS. 4A and 4B are cross-sectional views showing a fabrication processusing no etching stop layer;

FIG. 5 is a diagram showing an example of a dopant concentration, athickness and an Al composition ratio of each semiconductor layer of thelight-emitting thyristor according to the first embodiment;

FIG. 6 is a diagram showing the relationship between a P-type dopantconcentration of an active layer and the light amount of thelight-emitting thyristor;

FIG. 7 is a diagram showing the relationship between a multiplicationfactor (%) of a pn product as the product of an electron concentration nand a hole concentration p in the active layer and the thickness (nm) ofa P-type gate layer in a P-type emitter light emission type thyristor;

FIG. 8 is a diagram showing a composite function of currentamplification factors when a dopant concentration is changed;

FIG. 9 is a schematic cross-sectional view showing the cross-sectionalstructure of a light-emitting thyristor according to a firstmodification of the first embodiment;

FIG. 10 is a diagram showing an example of the dopant concentration, thethickness and the Al composition ratio of each semiconductor layer ofthe light-emitting thyristor according to the first modification of thefirst embodiment;

FIG. 11 is a diagram showing an example of the dopant concentration, thethickness and the Al composition ratio of each semiconductor layer of alight-emitting thyristor according to a second modification of the firstembodiment;

FIG. 12 is a schematic cross-sectional view showing the structure of alight-emitting thyristor according to a third modification of the firstembodiment;

FIG. 13 is a diagram showing an example of the dopant concentration, thethickness and the Al composition ratio of each semiconductor layer ofthe light-emitting thyristor according to the third modification of thefirst embodiment;

FIG. 14 is a schematic plan view showing the structure of alight-emitting thyristor according to a second embodiment of the presentinvention;

FIG. 15 is a schematic cross-sectional view showing the structure of thelight-emitting thyristor according to the second embodiment, namely, thecross-sectional structure at the line A-B-C in FIG. 14;

FIG. 16 is a diagram showing an example of the dopant concentration, thethickness and the Al composition ratio of each semiconductor layer ofthe light-emitting thyristor according to the second embodiment;

FIG. 17 is a schematic cross-sectional view showing the structure of alight-emitting thyristor according to a third embodiment of the presentinvention;

FIG. 18 is a diagram showing an example of the dopant concentration, thethickness and the Al composition ratio of each semiconductor layer ofthe light-emitting thyristor according to the third embodiment;

FIG. 19 is a schematic perspective view showing the structure of asubstrate unit as a principal part of an optical print head according toa fourth embodiment of the present invention;

FIG. 20 is a schematic cross-sectional view showing the structure of theoptical print head according to the fourth embodiment; and

FIG. 21 is a schematic cross-sectional view showing the structure of animage forming device according to a fifth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications will become apparent to those skilled in the art from thedetailed description.

Light-emitting thyristors, light-emitting element chips, optical printheads and image forming devices according to embodiments of the presentinvention will be described below with reference to drawings. In thedrawings, the same components are assigned the same referencecharacters. The following embodiments are just examples for the purposeof illustration and a variety of modifications are possible within thescope of the present invention. For example, configurations ofembodiments can be properly combined with each other.

In first to third embodiments, the light-emitting thyristors and thelight-emitting element chips will be described. The light-emittingelement chip includes one or more light-emitting thyristors. Thelight-emitting element chip may include a plurality of light-emittingthyristors arranged in a line. For example, the light-emitting elementchip includes a substrate part and a plurality of light-emittingthyristors arranged on the substrate part. The light-emitting elementchip may include a semiconductor integrated circuit part (referred toalso as a “drive IC part”) as a drive circuit for lighting up andextinguishing the plurality of light-emitting thyristors. Thelight-emitting element chip including the light-emitting thyristor andthe drive IC part is referred to also as a “semiconductor compositedevice”.

In a fourth embodiment, an optical print head including thelight-emitting element chip in any one of the first to third embodimentswill be described. The optical print head includes one or morelight-emitting element chips. The optical print head is an exposuredevice for forming an electrostatic latent image on the surface of aphotosensitive drum used as an image carrier of an image forming device.The optical print head may include a plurality of light-emitting elementchips arranged in a line.

In a fifth embodiment, an image forming device including the opticalprint head according to the fourth embodiment will be described. Theimage forming device is a device that forms an image made of adeveloping agent on a print medium by means of an electrophotographicprocess. The image forming device is a printer, a copy machine, afacsimile machine, a multi-function peripheral (MFP) or the like, forexample.

(1) First Embodiment (1-1) Configuration

FIG. 1 is a schematic plan view showing the structure of alight-emitting thyristor 10 according to the first embodiment. FIG. 1shows a semiconductor device 1000 including a plurality oflight-emitting thyristors 10. The semiconductor device 1000 is arrangedon a substrate part 101. As shown in FIG. 1, the substrate part 101includes a substrate 102 and a planarization layer 103 formed on thesubstrate 102, for example. A light-emitting element chip 100 includesthe substrate part 101 and the semiconductor device 1000 formed on thesubstrate part 101. The semiconductor device 1000 is referred to also asa “light-emitting element array” or a “light-emitting thyristor array”.

Further, the light-emitting element chip 100 is referred to also as a“light-emitting element array chip” or a “light-emitting thyristor arraychip”. Incidentally, an insulation film 71 (shown in FIG. 2) is notshown in FIG. 1 for easy understanding of the structure of thesemiconductor device 1000.

For example, a Si (silicon) substrate, an IC (integrated circuit)substrate, a glass substrate, a ceramic substrate, a plastic substrate,a metal plate or the like is usable as the substrate 102. In the firstembodiment, the substrate 102 is an IC substrate including a drive ICpart for driving the light-emitting thyristor as the three-terminallight-emitting element and an external connection pad 104 used forwiring to an external device.

The planarization layer 103 has a smooth surface on which thelight-emitting thyristors 10 are arranged. The planarization layer 103is an inorganic film or an organic film. In a case where a top surfaceof the substrate 102 is smooth, it is also possible to provide thesemiconductor device 1000 on the top surface of the substrate 102without providing the planarization layer 103.

The light-emitting thyristor 10 is formed on a growth substrate(referred to also as a “base material”) used as a manufacturingsubstrate, for example. In a case where the light-emitting thyristor 10is formed of an AlGaAs (aluminum gallium arsenide)-based semiconductormaterial, a GaAs (gallium arsenide) substrate can be used as the growthsubstrate. The light-emitting thyristor 10 is formed on the growthsubstrate by means of epitaxial growth, for example. The light-emittingthyristor 10 is formed by, for example, peeling off an epitaxial film,as a semiconductor thin film having a laminated structure ofsemiconductor layers, from the growth substrate, sticking the peeledepitaxial film on the surface of the planarization layer 103 on thesubstrate 102, and processing the epitaxial film by publicly knownphotolithography process and etching process. The epitaxial film placedon the surface of the planarization layer 103 is fixed to theplanarization layer 103 by intermolecular force or the like.

FIG. 2 is a schematic cross-sectional view showing the structure of thelight-emitting thyristor 10 according to the first embodiment, namely,the cross-sectional structure at the line A-B-C in FIG. 1. As shown inFIG. 2, the light-emitting thyristor 10 includes a first semiconductorlayer 1010 of a first conductivity type, a second semiconductor layer1020 of a second conductivity type different from the first conductivitytype arranged adjacent to the first semiconductor layer 1010, a thirdsemiconductor layer 1030 of the first conductivity type arrangedadjacent to the second semiconductor layer 1020, and a fourthsemiconductor layer 1040 of the second conductivity type arrangedadjacent to the third semiconductor layer 1030. In the first embodiment,the first conductivity type is the P type and the second conductivitytype is the N type.

Further, as shown in FIG. 2, the light-emitting thyristor 10 includes ananode electrode 61A as a first electrode electrically connected with thefirst semiconductor layer 1010, a gate electrode 51 as a secondelectrode electrically connected with the third semiconductor layer1030, and a cathode electrode 41K as a third electrode electricallyconnected with the fourth semiconductor layer 1040. The anode electrode61A is electrically connected with an anode terminal of the substratepart 101 by anode wiring 62A. The gate electrode 51 is electricallyconnected with a gate terminal 53 (shown in FIG. 1) of the substratepart 101 by gate wiring 52 (shown in FIG. 1). The cathode electrode 41Kis electrically connected with a cathode terminal 43K (shown in FIG. 1)of the substrate part 101 by cathode wiring 42K.

The P-type first semiconductor layer 1010 includes an anode layer 1011electrically connected with the anode electrode 61A and a P-type activelayer 1012 arranged adjacent to the anode layer 1011. Thus, a part ofthe first semiconductor layer 1010 is the active layer 1012 adjacent tothe second semiconductor layer 1020. The N-type second semiconductorlayer 1020 includes an N-type gate layer 1021. The P-type thirdsemiconductor layer 1030 includes a P-type gate layer 1031 and anetching stop layer 1032. The N-type fourth semiconductor layer 1040includes an N-type cathode layer 1041.

Semiconductor materials forming the first to fourth semiconductor layers1010, 1020, 1030 and 1040 are, for example, InP(indium-phosphorous)-based semiconductor materials, AlGaAs-basedsemiconductor materials, AlInGaP(aluminum-indium-gallium-phosphorous)-based semiconductor materials, orthe like.

In a case where AlGaAs-based semiconductor materials are used for thefirst to fourth semiconductor layers 1010, 1020, 1030 and 1040, theanode layer 1011 is, for example, a P-type Al_(0.25)Ga_(0.75)As layer,the active layer 1012 is, for example, a P-type Al_(0.15)Ga_(0.85)Aslayer, the N-type gate layer 1021 is, for example, an N-typeAl_(0.15)Ga_(0.85)As layer, the P-type gate layer 1031 is, for example,a P-type Al_(0.15)Ga_(0.85)As layer, and the cathode layer 1041 is, forexample, an N-type Al_(0.25)Ga_(0.75)As layer. The etching stop layer1032 is, for example, a P-type In_(0.49)Ga_(0.51)P layer.

The etching stop layer 1032 is used in a fabrication process ofsemiconductor layers. FIGS. 3A to 3C are cross-sectional views showing afabrication process using the etching stop layer 1032. First, alaminated structure of the P-type gate layer 1031, the etching stoplayer 1032 and the cathode layer 1041 is etched by using a resist as amask as shown in FIG. 3A, and thereby the structure shown in FIG. 3B isobtained.

Subsequently, the etching stop layer 1032 is etched by using etchingliquid for the etching stop layer 1032, and thereby the structure shownin FIG. 3C is obtained. In this case, the thickness T3 of the P-typegate layer 1031 can be set thin since the etching liquid for the etchingstop layer 1032 does not etch the P-type gate layer 1031.

FIGS. 4A and 4B are cross-sectional views showing a fabrication processusing no etching stop layer. First, a laminated structure of a P-typegate layer 1031 c and the cathode layer 1041 is etched by using a resistas a mask as shown in FIG. 4A, and thereby the structure shown in FIG.4B is obtained. In this case, the thickness T3 c of the P-type gatelayer 1031 c is designed to be thicker than the thickness T3 in FIG. 3Csince etching liquid for the cathode layer 1041 etches the P-type gatelayer 1031 c.

Each of the anode electrode 61A, the gate electrode 51 and the cathodeelectrode 41K shown in FIG. 1 and FIG. 2 is, for example, a metal or analloy which is capable of forming an ohmic contact with AlGaAs or alaminated structure of some of such a metal and an alloy. The metal is,for example, Ti (titanium), Pt (platinum), Au (gold), Ge (germanium), Ni(nickel), Zn (zinc) or the like. The alloy is alloy of some of thesemetals. The laminated structure is a laminated structure of metals, alaminated structure of alloys, or a laminated structure of metal(s) andalloy(s). The insulation film 71 is an inorganic insulation film such asa SiN (silicon nitride) film or a SiO₂ (silicon dioxide) film, or anorganic insulation film such as a polyimide film.

In the first embodiment, a dopant concentration Nae of the active layer1012 is higher than or equal to a dopant concentration Npg of the thirdsemiconductor layer 1030. Further, the band gap BGae of the active layer1012 is narrower than or equal to the band gap BGng of the secondsemiconductor layer 1020 and narrower than or equal to the band gap BGpgof the third semiconductor layer 1030. Furthermore, the thickness T3 ofthe third semiconductor layer 1030 is thinner than the thickness T2 ofthe second semiconductor layer 1020. Moreover, a dopant concentrationNng of the second semiconductor layer 1020 is lower than the dopantconcentration Npg of the third semiconductor layer 1030.

Namely, the light-emitting thyristor according to the first embodimentsatisfies the following conditions (1) to (5):Nae≥Npg  (1)BGae≤BGng  (2)BGae≤BGpg  (3)T2>T3  (4)Nng<Npg  (5)

The conditions (2) and (3) are equivalent to a condition that an Alcomposition ratio Cae of the active layer 1012 is lower than or equal toan Al composition ratio Cng of the second semiconductor layer 1020 andlower than or equal to an Al composition ratio Cpg of the thirdsemiconductor layer 1030. Thus, the conditions (2) and (3) can bereplaced with the following conditions (6) and (7):Cae≤Cng  (6)Cae≤Cpg  (7)

FIG. 5 is a diagram showing an example of the dopant concentration(cm⁻³), the thickness (nm) and the Al (aluminum) composition ratio ofeach semiconductor layer of the light-emitting thyristor 10. The bandgap of AlGaAs is proportional to the Al composition ratio of AlGaAs.

In the light-emitting thyristor 10 according to the first embodiment,the Al composition ratio Cae of the active layer 1012 is set equal tothe Al composition ratio Cpg of the P-type gate layer 1031 and the Alcomposition ratio Cng of the N-type gate layer 1021, and set lower thanthe Al composition ratio C11 of the anode layer 1011 and the Alcomposition ratio C41 of the cathode layer 1041. Namely, in thelight-emitting thyristor 10 according to the first embodiment, thefollowing condition (8) holds:Cae=Cpg=Cng<C11 (or C41)  (8)

Further, to reduce contact resistance, the dopant concentration N11 ofthe anode layer 1011 is set at 5×10¹⁸ cm⁻³ (=5E+18 cm⁻³) and the dopantconcentration N41 of the cathode layer 1041 is set at 1.5×10¹⁸ cm⁻³(=1.5E+18 cm⁻³).

FIG. 6 is a diagram showing the relationship between a P-type dopantconcentration (cm⁻³) of the active layer 1012 and the light amount ofthe light-emitting thyristor 10. It can be seen from FIG. 6 that a lightamount multiplication factor (%) representing the change in the amountof light emitted from the light-emitting thyristor 10 increasesproportionally to the dopant concentration Nae (cm⁻³) of the activelayer 1012. Therefore, in the first embodiment, the dopant concentrationNae of the active layer 1012 is set at 1×10¹⁹ cm⁻³ (=1E+19 cm⁻³) asshown in FIG. 5.

FIG. 7 is a diagram showing the relationship between a multiplicationfactor (%) of a pn product as the product of an electron concentration nand a hole concentration p in the active layer 1012 and the thicknessTpg (nm) of the P-type gate layer 1031 in the light-emitting thyristor10 according to the first embodiment as a P-type emitter light emissiontype thyristor. In the first embodiment, the thickness Tpg is thethickness T3 shown in FIG. 5. The light emission in the light-emittingthyristor is caused by recombination of an electron and a hole, and therecombination probability is proportional to the multiplication factorof the pn product. The pn product multiplication factor in FIG. 7 doesnot change even if the P-type dopant concentration Npg of the P-typegate layer 1031 and the P-type dopant concentration Nae of the activelayer 1012 are changed. Thus, in a case where the light-emittingthyristor 10 is provided with the active layer 1012 as a P-type emitter,it can be understood that the light amount increases proportionally tothe multiplication factor of the pn product if the thickness Tpg (=T3)of the P-type gate layer 1031 is set thin. For this reason, thethickness Tpg of the P-type gate layer 1031 is 150 nm. Accordingly, inorder to secure the stability of the fabrication process, the etchingstop layer 1032 is provided immediately above the P-type gate layer1031.

Subsequently, when the thickness Tpg of the P-type gate layer 1031 is150 nm, the dopant concentration Npg of the P-type gate layer which cansecure sufficient withstand voltage performance is determined. Thewidths of depletion layers in one PN junction can be calculated by usingthe following expressions (9) and (10):

$\begin{matrix}{X_{n} = \sqrt{\frac{2ɛ{N_{A}\left( {V_{bi} - V} \right)}}{q{N_{D}\left( {N_{D} + N_{A}} \right)}}}} & (9) \\{X_{P} = \sqrt{\frac{2ɛ{N_{D}\left( {V_{bi} - V} \right)}}{q{N_{a}\left( {N_{D} + N_{A}} \right)}}}} & (10)\end{matrix}$

Here, X_(n) and X_(p) represent the widths of the depletion layersextending respectively on an N-type layer's side and a P-type layer'sside of the PN junction, N_(D) and N_(A) represent a donor concentrationand an acceptor concentration, c represents a dielectric constant, qrepresents the elementary charge, V_(bi) represents built-in potential,and V represents external voltage. The external voltage is positive whenit is in the forward direction of the PN junction.

The expression (9) indicates that the width X_(n) of the depletion layerextending on the N-type layer's side increase with the decrease in theconcentration of N-type impurities as donors and with the increase inthe concentration of P-type impurities as acceptors in the N-type layer,and the expression (10) indicates an inverse phenomenon with regard toX_(p). Further, in the expressions (9) and (10), the widths X_(n) andX_(p) of the depletion layers increase when reverse direction voltage isincreased. For example, when forward direction voltage is appliedbetween the anode electrode 61A and the gate electrode 51 in FIG. 2,voltage in the reverse direction is applied to the PN junction betweenthe P-type gate layer and the N-type gate layer. When a depletion layerexpanding due to the voltage in the reverse direction connects with adepletion layer in the vicinity of the junction and on the oppositeside, the punch-through occurs and electrical conduction is establishedbetween the anode and the gate. Therefore, in order to secure sufficientwithstand voltage performance, the thickness Tpg (=T3) is set greaterthan the sum total of the width of a depletion layer on a P-type gatelayer's side in a PN junction between the P-type gate layer and theN-type gate layer when forward direction voltage is applied between theanode and the gate and the width of a depletion layer on a P-type gatelayer's side in a PN junction between the P-type gate layer and theN-type cathode layer when the voltage between the P-type gate layer andthe N-type cathode layer is 0 V. For example, in consideration of thedopant concentration Nng being as high as 1×10¹⁸ cm⁻³ and variationsoccurring at the time of the growth of the semiconductor layers, thedopant concentration Npg is set at 1×10¹⁸ cm⁻³ as the dopantconcentration of the P-type gate layer 1031 whose withstand voltage is 8V or higher.

Finally, the dopant concentration Npg and the thickness Tpg (=T3) of theP-type gate layer 1031, which is capable of reducing breakover voltageVb necessary for turning on the light-emitting thyristor 10 whilesufficient withstand voltage performance is secured, are determined. Thebreakover voltage Vb is voltage necessary for turning on thelight-emitting thyristor 10. The dopant concentration N_(B) of a baselayer (namely, a gate layer in the light-emitting thyristor 10) isincreased or a base width W_(B) is increased, and thereby currentamplification factors β₁ and β₂ decrease. As the current amplificationfactors β₁ and β₂ decrease, the breakover voltage Vb necessary forturning on the light-emitting thyristor 10 increases. Since thebreakover voltage Vb has to be set lower than drive voltage suppliedfrom a drive circuit for driving the light-emitting thyristor 10, thecurrent amplification factors β₁ and β₂ have to be set rather high. Inregard to a PNP transistor part and an NPN transistor part in thelight-emitting thyristor 10, the current amplification factors β₁ and β₂can be respectively determined according to the following expressions(11) and (12):

$\begin{matrix}{\beta_{1} = \frac{1}{\frac{W_{B}^{2}}{L_{B}^{2}} + \frac{D_{B}N_{B}W_{B}}{D_{E}N_{E}W_{E}}}} & (11) \\{\beta_{2} = \frac{1}{\frac{W_{B}^{2}}{L_{B}^{2}} + \frac{D_{B}N_{B}W_{B}}{D_{E}N_{E}W_{E}}}} & (12)\end{matrix}$

Here, the base width W_(B) in the expressions (11) and (12) is a valueobtained by subtracting the sum total of the widths X_(n) and X_(p) ofthe depletion layers from the thickness Tng (=T2) of the N-type gatelayer 1021. Thus, if the thickness Tng of the N-type gate layer 1021 isset at a constant multiple of the width of a depletion layer, the basewidth W_(B) is determined from the dopant concentration Nng. It has beenconfirmed by experiments that the breakover voltage Vb has negativecorrelation with each of the current amplification factors β₁ and β₂.Further, the breakover voltage Vb is considered to have negativecorrelation with a composite function β of the current amplificationfactors. Thus, the composite function β is considered to be obtainedaccording to the following expression (13):β=αβ₁β₂  (13)Here, the coefficient a is an arbitrary constant. In this case, a resultwas obtained as shown in FIG. 8 in which the composite function β of thecurrent amplification factors had a local maximum value with regard to achange of the dopant concentration Nng of the N-type gate layer 1021.Accordingly, the dopant concentration Nng of the N-type gate layer 1021is set at 5×10¹⁷ cm⁻³ and the thickness Tng (=T2) of the N-type gatelayer 1021 is set at 300 nm as shown in FIG. 5. Incidentally, Nk (N41 inFIG. 5) represents the dopant concentration of the cathode layer 1041and Nae represents the dopant concentration of the active layer 1012.

The light-emitting thyristor 10 according to the first embodiment is notlimited to the structure shown in FIG. 5. In the light-emittingthyristor 10 according to the first embodiment, if the followingconditions (14) to (19) are satisfied, the composite function β of thecurrent amplification factors can be made larger compared to that inconventional technology while withstand voltage of 8 V or higher issecured.150 nm≤Tpg (=T3)≤180 nm  (14)270 nm≤Tng (=T2)≤330 nm  (15)1.2×10¹⁸ cm⁻³ ≤Nk≤1.8×10¹⁸ cm⁻³  (16)8×10¹⁷ cm⁻³≤Npg≤1.2×10¹⁸ cm⁻³  (17)4×10¹⁷ cm⁻³≤Nng≤6×10¹⁷ cm⁻³  (18)1.2×10¹⁸ cm⁻³≤Nae≤1.5×10¹⁹ cm⁻³  (19)

The light-emitting thyristor 10 according to the first embodimentsatisfies the following conditions (20) and (21):Tpg<Tng  (20)Nng<Npg≤Nae  (21)

(1-2) Operation

In the light-emitting thyristor 10 according to the first embodiment, anelectric current is sent from the gate electrode 51 to the cathodeelectrode 41K and thereby electrical conduction is established betweenthe anode electrode 61A and the cathode electrode 41K. In this case, ahole and an electron recombine with each other in the P-type activelayer 1012. At that time, although similar recombination occurs also inthe N-type gate layer 1021 and the P-type gate layer 1031, therecombination occurs in the active layer 1012 with high probabilitysince the dopant concentration Nng in the N-type gate layer 1021 is setlow and the thickness Tpg of the P-type gate layer 1031 is set thin.Light generated by the recombination is emitted through the cathodelayer 1041.

(1-3) Effect

In the light-emitting thyristor 10 according to the first embodiment,the composite function β of the current amplification factors becomeslarge while sufficient withstand voltage performance is secured by thecalculations described above, and thereby the breakover voltage Vb canbe reduced. Further, since the N-type gate layer 1021 and the P-typegate layer 1031 serve also as absorptive layers that absorb the lightgenerated in the active layer 1012, light extraction efficiency can beincreased by reducing the thicknesses of these layers. With theabove-described features, in the light-emitting thyristor 10 accordingto the first embodiment, the breakover voltage Vb decreases and luminousefficiency increases.

Further, in the case of the comparative example using no etching stoplayer 1032 in the fabrication process, the thickness of the P-type gatelayer 1031 has to be set as thick as approximately 400 nm and thebreakover voltage Vb also increases to 5.44 V. In contrast, in thelight-emitting thyristor 10 according to the first embodiment, since theetching stop layer 1032 is used in the fabrication process, thethickness of the P-type gate layer 1031 can be set as thin as 150 nm,the breakover voltage Vb can be lowered to 2.56 V, and the emissionlight amount can be increased to 152% relative to the comparativeexample.

Incidentally, the above description has been given of the example inwhich semiconductor layers are stacked upward from the substrate part101 in the order of PNPN, the etching stop layer 1032 is providedimmediately above the P-type gate layer 1031, the Al composition ratiosof the active layer 1012, the N-type gate layer 1021 and the P-type gatelayer 1031 are equal to each other, and the gate electrode 51 isconnected to the P-type gate layer 1031. However, it is also possible toemploy a different structure such as a structure including no etchingstop layer 1032.

(1-4) First Modification of First Embodiment

FIG. 9 is a schematic cross-sectional view showing the cross-sectionalstructure of a light-emitting thyristor 11 according to a firstmodification of the first embodiment. FIG. 10 is a diagram showing anexample of the dopant concentration (cm⁻³), the thickness (nm) and theAl composition ratio of each semiconductor layer of the light-emittingthyristor 11. The light-emitting thyristor 11 and a semiconductor device1100 differ from the light-emitting thyristor 10 and the semiconductordevice 1000 in that a third semiconductor layer 1030 a includes a P-typegate layer 1031 a and an etching stop layer 1032 a and in that the gateelectrode 51 is connected to the N-type gate layer 1021. In regard tothe other features, the light-emitting thyristor 11 and thesemiconductor device 1100 are the same as the light-emitting thyristor10 and the semiconductor device 1000.

(1-5) Second Modification of First Embodiment

FIG. 11 is a diagram showing an example of the dopant concentration(cm⁻³), the thickness (nm) and the Al composition ratio of eachsemiconductor layer of a light-emitting thyristor 10 a according to asecond modification of the first embodiment. The light-emittingthyristor 10 a differs from the light-emitting thyristor 10 in that theAl composition ratio Cpg of the P-type gate layer 1031 and the Alcomposition ratio Cng of the N-type gate layer 1021 are higher than theAl composition ratio Cae of an active layer 1012 a. Even in a case wherethe Al composition ratios Cng and Cpg of the N-type gate layer 1021 andthe P-type gate layer 1031 are set higher than the Al composition ratioCae of the active layer as shown in FIG. 11, the changes in the built-inpotential V_(bi) and the widths X_(n) and X_(p) of the depletion layersat the junction between semiconductor layers are minute and sufficientwithstand voltage can be secured within the ranges of the thicknessesand the dopant concentrations of the semiconductor layers specified bythe aforementioned conditions (14) to (21). In addition, since thetransmittance of light increases by increasing the Al composition ratiosCng and Cpg, the light extraction efficiency increases and an advantageof further increasing the light amount is obtained. Except for theabove-described features, the light-emitting thyristor 10 a is the sameas the light-emitting thyristor 10.

(1-6) Third Modification of First Embodiment

FIG. 12 is a schematic cross-sectional view showing the cross-sectionalstructure of a light-emitting thyristor 12 according to a thirdmodification of the first embodiment. FIG. 13 is a diagram showing anexample of the dopant concentration (cm⁻³), the thickness (nm) and theAl composition ratio of each semiconductor layer of the light-emittingthyristor 12. The light-emitting thyristor 12 differs from thelight-emitting thyristor 10 in that the fourth semiconductor layer 1040,the third semiconductor layer 1030, the second semiconductor layer 1020and the first semiconductor layer 1010 are stacked on the substrate part101 in this order from the side of the substrate part 101 and inincluding an anode electrode 41A, anode wiring 42A, a cathode electrode61K and cathode wiring 62K. In the light-emitting thyristor 12, sinceonly the anode layer 1011 exists on top of the active layer 1012,absorption of light traveling upward is reduced, the light extractionefficiency increases, and the advantage of further increasing the lightamount is obtained. Except for the above-described features, thelight-emitting thyristor 12 is the same as the light-emitting thyristor10.

(2) Second Embodiment (2-1) Configuration

FIG. 14 is a schematic plan view showing the structure of alight-emitting thyristor 20 according to a second embodiment of thepresent invention. FIG. 15 is a schematic cross-sectional view showingthe structure of the light-emitting thyristor 20. As shown in FIG. 14and FIG. 15, the light-emitting thyristor 20 includes a firstsemiconductor layer 2010 of a first conductivity type, a secondsemiconductor layer 2020 of a second conductivity type different fromthe first conductivity type arranged adjacent to the first semiconductorlayer 2010, a third semiconductor layer 2030 of the first conductivitytype arranged adjacent to the second semiconductor layer 2020, and afourth semiconductor layer 2040 of the second conductivity type arrangedadjacent to the third semiconductor layer 2030. In the secondembodiment, the first conductivity type is the N type and the secondconductivity type is the P type.

Further, as shown in FIG. 15, the light-emitting thyristor 20 includes acathode electrode 61K as a first electrode electrically connected withthe first semiconductor layer 2010, a gate electrode 51 as a secondelectrode electrically connected with the third semiconductor layer2030, and an anode electrode 41A as a third electrode electricallyconnected with the fourth semiconductor layer 2040. The cathodeelectrode 61K is electrically connected with a cathode terminal of thesubstrate part 101 by cathode wiring 62K. The gate electrode 51 iselectrically connected with a gate terminal 53 (shown in FIG. 14) of thesubstrate part 101 by gate wiring 52 (shown in FIG. 14). The anodeelectrode 41A is electrically connected with an anode terminal 43A(shown in FIG. 14) of the substrate part 101 by anode wiring 42A.

The N-type first semiconductor layer 2010 includes a cathode layer 2011and an N-type active layer 2012 arranged adjacent to the cathode layer2011. Thus, a part of the first semiconductor layer 2010 is the activelayer 2012 adjacent to the second semiconductor layer 2020. The P-typesecond semiconductor layer 2020 includes a P-type gate layer 2021. TheN-type third semiconductor layer 2030 includes an N-type gate layer 2031and an etching stop layer 2032. The P-type fourth semiconductor layer2040 includes a P-type anode layer 2041.

In a case where AlGaAs-based semiconductor materials are used for thefirst to fourth semiconductor layers 2010, 2020, 2030 and 2040, thecathode layer 2011 is, for example, an N-type Al_(0.25)Ga_(0.75)Aslayer, the active layer 2012 is, for example, an N-typeAl_(0.15)Ga_(0.85)As layer, the P-type gate layer 2021 is, for example,a P-type Al_(0.15)Ga_(0.85)As layer, the N-type gate layer 2031 is, forexample, an N-type Al_(0.15)Ga_(0.85)As layer, and the anode layer 2041is, for example, a P-type Al_(0.25)Ga_(0.75)As layer. The etching stoplayer 2032 is, for example, an N-type In_(0.49)Ga_(0.51)P layer.

In the second embodiment, the dopant concentration Nae of the activelayer 2012 is higher than or equal to the dopant concentration Nng ofthe third semiconductor layer 2030. Further, the band gap BGae of theactive layer 2012 is narrower than or equal to the band gap BGpg of thesecond semiconductor layer 2020 and narrower than or equal to the bandgap BGng of the third semiconductor layer 2030. Furthermore, thethickness T3 of the third semiconductor layer 2030 is thinner than thethickness T2 of the second semiconductor layer 2020. Moreover, thedopant concentration Npg of the second semiconductor layer 2020 is lowerthan the dopant concentration Nng of the third semiconductor layer 2030.

Namely, the light-emitting thyristor according to the second embodimentsatisfies the following conditions (1a), (2) to (4) and (5a):Nae≥Nng  (1a)BGae≤BGng  (2)BGae≤BGpg  (3)T2>T3  (4)Npg<Nng  (5a)

The conditions (2) and (3) are equivalent to a condition that an Alcomposition ratio Cae of the active layer 2012 is lower than or equal toan Al composition ratio Cpg of the second semiconductor layer 2020 andlower than or equal to an Al composition ratio Cng of the thirdsemiconductor layer 2030. Thus, the conditions (2) and (3) can bereplaced with the following conditions (6) and (7):Cae≤Cng  (6)Cae≤Cpg  (7)

FIG. 16 is a diagram showing an example of the dopant concentration(cm⁻³), the thickness (nm) and the Al composition ratio of eachsemiconductor layer of the light-emitting thyristor 20.

In the light-emitting thyristor 20 according to the second embodiment,the Al composition ratio Cae of the active layer 2012 is set equal tothe Al composition ratio Cng of the N-type gate layer 2031 and the Alcomposition ratio Cpg of the P-type gate layer 2021, and set lower thanthe Al composition ratio C11 of the cathode layer 2011 and the Alcomposition ratio C41 of the anode layer 2041. Namely, in thelight-emitting thyristor 20 according to the second embodiment, thefollowing condition (8) holds:Cae=Cpg=Cng<C11 (or C41)  (8)

The light-emitting thyristor 20 according to the second embodiment isnot limited to the structure shown in FIG. 16. In the light-emittingthyristor 20 according to the second embodiment, if the followingconditions (22) to (29) are satisfied, the composite function β of thecurrent amplification factors can be made larger compared to that inconventional technology while withstand voltage of 8 V or higher issecured.180 nm≤Tng (=T3)≤220 nm  (22)270 nm≤Tpg (=T2)≤330 nm  (23)4×10¹⁸ cm⁻³≤Na≤6×10¹⁸ cm⁻³  (24)7×10¹⁷ cm⁻³≤Nng≤1×10¹⁸ cm⁻³  (25)4×10¹⁷ cm⁻³≤Npg≤6×10¹⁷ cm⁻³  (26)1×10¹⁸ cm⁻³≤Nae≤1.5×10¹⁸ cm⁻³  (27)

Within the above ranges, the composite function β of the currentamplification factors can be made larger compared to that inconventional technology while withstand voltage of 8 V or higher issecured.

The light-emitting thyristor 20 according to the second embodimentsatisfies the following conditions (28) and (29):Tng<Tpg  (28)Npg<Nng≤Nae  (29)

(2-2) Operation

In the light-emitting thyristor 20 according to the second embodiment,an electric current is sent from the anode electrode 41A to the N-typegate electrode 51 and thereby electrical conduction is establishedbetween the anode electrode 41A and the cathode electrode 61K. In thiscase, a hole and an electron recombine with each other in the N-typeactive layer 2012. At that time, although similar recombination occursalso in the P-type gate layer 2021 and the N-type gate layer 2031, therecombination occurs in the active layer 2012 with high probabilitysince the dopant concentration Npg in the P-type gate layer 2021 is setlow and the thickness Tng (=T3) of the N-type gate layer 2031 is setthin. Light generated by the recombination is emitted through the anodelayer 2041.

(2-3) Effect

In the light-emitting thyristor 20 according to the second embodiment,the composite function β of the current amplification factors becomeslarge while sufficient withstand voltage performance is secured by thecalculations described above, and thereby the breakover voltage Vb canbe reduced. Further, since the P-type gate layer 2021 and the N-typegate layer 2031 serve also as absorptive layers that absorb the lightgenerated in the active layer 2012, the light extraction efficiency canbe increased by reducing the thicknesses of these layers. With theabove-described features, in the light-emitting thyristor 20 accordingto the second embodiment, the breakover voltage Vb decreases and theluminous efficiency increases.

Further, since the etching stop layer 2032 is used in the fabricationprocess, the thickness of the N-type gate layer 2031 can be set thin andthe breakover voltage Vb can be lowered.

Incidentally, the above description has been given of the example inwhich semiconductor layers are stacked upward from the substrate part101 in the order of NPNP, the etching stop layer 2032 is providedimmediately above the N-type gate layer 2031, the Al composition ratiosof the active layer 2012, the P-type gate layer 2021 and the N-type gatelayer 2031 are equal to each other, and the gate electrode 51 isconnected to the N-type gate layer 2031. However, it is also possible toemploy a different structure such as a structure including no etchingstop layer 2032.

(3) Third Embodiment (3-1) Configuration

FIG. 17 is a schematic cross-sectional view showing the structure of alight-emitting thyristor 30 according to a third embodiment. FIG. 18 isa diagram showing an example of the dopant concentration, the thicknessand the Al composition ratio of each semiconductor layer of thelight-emitting thyristor 30. The light-emitting thyristor 30 accordingto the third embodiment differs from the light-emitting thyristor 10according to the first embodiment in including a hole barrier layer 3021a and an electron barrier layer 3012 a having higher Al compositionratios than the anode layer and the cathode layer. First to fourthsemiconductor layers 3010, 3020, 3030 and 3040 of the light-emittingthyristor 30 correspond to the first to fourth semiconductor layers1010, 1020, 1030 and 1040 of the light-emitting thyristor 10. Thestructure of an anode layer 3011, an active layer 3012, an N-type gatelayer 3021, an etching stop layer 3032 and a cathode layer 3041 of thelight-emitting thyristor 30 is respectively the same as the structure ofthe anode layer 1011, the active layer 1012, the N-type gate layer 1021,the etching stop layer 1032 and the cathode layer 1041 of thelight-emitting thyristor 10.

In a case where AlGaAs-based semiconductor materials are used for thefirst to fourth semiconductor layers 3010, 3020, 3030 and 3040, theanode layer 3011 is, for example, a P-type Al_(0.25)Ga_(0.75)As layer,the electron barrier layer 3012 a is, for example, a P-typeAl_(0.40)Ga_(0.60)As layer, the active layer 3012 is, for example, aP-type Al_(0.15)Ga_(0.85)As layer, the hole barrier layer 3021 a is, forexample, an N-type Al_(0.40)Ga_(0.60)As layer, the N-type gate layer3021 is, for example, an N-type Al_(0.15)Ga_(0.85)As layer, the P-typegate layer 3031 is, for example, a P-type Al_(0.15)Ga_(0.85)As layer,and the cathode layer 3041 is, for example, an N-typeAl_(0.25)Ga_(0.75)As layer. The etching stop layer 3032 is, for example,a P-type In_(0.49)Ga_(0.51)P layer.

In the third embodiment, the base layer of the PNP transistor in thelight-emitting thyristor 30 is separated into two layers: the holebarrier layer 3021 a and the N-type gate layer 3021, and thus the sumtotal of the thicknesses of the two layers is set at the thickness Tngin the first embodiment. Further, the dopant concentration of theelectron barrier layer 3012 a is equal to the dopant concentration Naeof the active layer 3012, and the dopant concentration of the holebarrier layer 3021 a is equal to the dopant concentration Nng of theN-type gate layer 3021.

The light-emitting thyristor 30 according to the third embodiment is notlimited to the structure shown in FIG. 18. In the light-emittingthyristor 20 according to the second embodiment, if the followingconditions (14) to (19) are satisfied, the composite function β of thecurrent amplification factors can be made larger compared to that inconventional technology while securing withstand voltage of 8 V orhigher is secured.150 nm≤Tpg (=T3)≤180 nm  (14)270 nm≤Tng (=T2)≤330 nm  (15)1.2×10¹⁸ cm⁻³ ≤Nk<1.8×10¹⁸ cm⁻³  (16)8×10¹⁷ cm⁻³≤Npg≤1.2×10¹⁸ cm⁻³  (17)4×10¹⁷ cm⁻³≤Nng≤6×10¹⁷ cm⁻³  (18)1.2×10¹⁸ cm⁻³≤Nae≤1.5×10¹⁹ cm⁻³  (19)

Within the above ranges, the composite function β of the currentamplification factors can be made larger compared to that inconventional technology while withstand voltage of 8 V or higher issecured.

The light-emitting thyristor 30 according to the third embodimentsatisfies the following conditions (20) and (21):Tpg<Tng  (20)Nng<Npg≤Nae  (21)

The reason for providing the hole barrier layer 3021 a having a high Alcomposition ratio Cbh and a wide band gap between the P-type activelayer 3012 and the N-type gate layer 3021 is that an energy barrieroccurs against holes in the active layer 3012 moving towards the cathodelayer 3041 in a case where the band gap of the hole barrier layer 3021 ais wider than the band gap of the cathode layer 3041. Namely, since thehole barrier layer 3021 a with the wide band gap has a function as abarrier layer limiting the passage of holes, it is possible to inhibitholes from leaking out from the active layer 3012. Accordingly, thedecrease in the amount of holes in the active layer 3012 is inhibitedand the occurrence probability of the recombination of a hole and anelectron in the active layer 3012 becomes high.

The reason for providing the electron barrier layer 3012 a having a highAl composition ratio Cbe and a wide band gap between the P-type activelayer 3012 and the anode layer 3011 is that the band gap of the electronbarrier layer 3012 a works as a barrier layer against electrons in theP-type active layer 3012 advancing towards the electron barrier layer3012 a, electron confinement effect in the active layer 3012 can beenhanced, and the recombination in the active layer 3012 can beincreased.

(3-2) Operation

The light-emitting thyristor 30 according to the third embodimentoperates similarly to the light-emitting thyristor 10 according to thefirst embodiment.

(3-3) Effect

In the light-emitting thyristor 30 according to the third embodiment,similarly to the light-emitting thyristor 10 according to the firstembodiment, the composite function β of the current amplificationfactors becomes large while sufficient withstand voltage performance issecured, and thereby the breakover voltage Vb can be reduced.

Further, the light extraction efficiency increases since the P-type gatelayer 3031 absorbing the light from the active layer 3012 is designed tobe thin.

Furthermore, energy barriers are formed by the electron barrier layer3012 a and the hole barrier layer 3021 a which are adjacent to theactive layer 3012, carriers are confined in the active layer 3012, andthereby the recombination in the active layer 3012 is promoted andinternal quantum efficiency increases. Accordingly, the light amountincreases further compared to the first embodiment.

Incidentally, the above description has been given of the example inwhich semiconductor layers are stacked upward from the substrate part101 in the order of PNPN. However, it is also possible to employ adifferent structure such as a structure including no etching stop layer3032. Further, it is also possible to incorporate one or more of thevarious types of structures described in the first and secondembodiments.

(4) Fourth Embodiment

FIG. 19 is a schematic perspective view showing the structure of aprincipal part of an optical print head according to a fourthembodiment. As shown in FIG. 19, a substrate unit as the principal partincludes a printed wiring board 801 as a mounting substrate and aplurality of light-emitting element chips 100 arranged like an array.The light-emitting element chips 100 are fixed on the printed wiringboard 801 by using thermosetting resin or the like. Electrode pads 152of the light-emitting element chips 100 for external connection andconnection pads 802 of the printed wiring board 801 are electricallyconnected to each other by bonding wires 803. The printed wiring board801 may also be equipped with various types of wiring patterns,electronic components, connectors, etc. It is also possible to employone of other light-emitting element chips described in the first tothird embodiments instead of the light-emitting element chip 100.

FIG. 20 is a schematic cross-sectional view showing the structure of theoptical print head 800 according to the fourth embodiment. The opticalprint head 800 is an exposure device of an electrophotographic printeras an image forming device of the electrophotographic type. As shown inFIG. 20, the optical print head 800 includes a base member 811, theprinted wiring board 801, the light-emitting element chips 100, a lensarray 813 including a plurality of upright isometric imaging lenses, alens holder 814, and clampers 815 as spring members. The base member 811is a member for fixing the printed wiring board 801. Side faces of thebase member 811 are provided with opening parts 812 to be used forfixing the printed wiring board 801 and the lens holder 814 to the basemember 811 by use of the clampers 815. The lens holder 814 is formed byinjection molding of organic polymeric material or the like, forexample. The lens array 813 is a set of optical lenses focusing lightemitted from the light-emitting element chips 100 on a photosensitivedrum as an image carrier. The lens holder 814 holds the lens array 813at a prescribed position with respect to the base member 811. Theclampers 815 clamp and hold components via the opening parts 812 of thebase member 811 and opening parts of the lens holder 814.

In the optical print head 800, the light-emitting thyristors of thelight-emitting element chips 100 selectively emit light according toprint data, and the light emitted from the light-emitting thyristors isfocused on the uniformly charged photosensitive drum by the lens array813. By this process, an electrostatic latent image is formed on thephotosensitive drum, and thereafter, an image made of a developing agentis formed (printed) on a print medium (sheet) by a development process,a transfer process and a fixation process.

As described above, since the optical print head 800 according to thefourth embodiment includes the light-emitting element chips 100 of lowvariations in light emission intensity, print quality can be improved byinstalling the optical print head 800 in an image forming device.

(5) Fifth Embodiment

FIG. 21 is a schematic cross-sectional view showing the structure of animage forming device 900 according to a fifth embodiment. The imageforming device 900 is a color printer using an electrophotographicprocess, for example.

As shown in FIG. 21, principal components of the image forming device900 include image formation sections (i.e., process units) 910K, 910Y,910M and 910C for forming a toner image (i.e., a developing agent image)on a record medium P such as a sheet of paper by an electrophotographicprocess, a medium supply section 920 for supplying the record medium Pto the image formation sections 910K, 910Y, 910M and 910C, and aconveyance section 930 for conveying the record medium P. Further, theimage forming device 900 includes transfer rollers 940K, 940Y, 940M and940C as transfer sections arranged respectively corresponding to theimage formation sections 910K, 910Y, 910M and 910C, a fixation device950 for fixing the toner images transferred onto the record medium P,and a guide 926 and an ejection roller pair 925 as a medium ejectionsection for ejecting the record medium P after passing through thefixation device 950 to the outside of a housing of the image formingdevice 900. The number of image formation sections included in the imageforming device 900 may also be three or less or five or more. Further,the image forming device 900 can also be a monochrome printer, in whichthe number of image formation sections is one, as long as the imageforming device 900 is a device forming an image on a record medium P bymeans of the electrophotographic process.

As shown in FIG. 21, the medium supply section 920 includes a mediumcassette 921, a hopping roller 922 for drawing out the record media Ploaded in the medium cassette 921 sheet by sheet, a roller pair 923 forconveying the record medium P drawn out of the medium cassette 921, aguide 970 for guiding the record medium P, and a registration roller anda pinch roller 924 for correcting skew of the record medium P.

The image formation sections 910K, 910Y, 910M and 910C respectively forma black (K) toner image, a yellow (Y) toner image, a magenta (M) tonerimage and a cyan (C) toner image on the record medium P. The imageformation sections 910K, 910Y, 910M and 910C are arranged side by sidealong a medium conveyance path from an upstream side to a downstreamside (i.e., from right to left in FIG. 21) in a medium conveyancedirection. Each of the image formation sections 910K, 910Y, 910M and910C may also be configured as an attachable and detachable unit. Theimage formation sections 910K, 910Y, 910M and 910C have basically thesame structure as each other except for the difference in the color ofthe stored toner.

The image formation sections 910K, 910Y, 910M and 910C respectivelyinclude optical print heads 911K, 911Y, 911M and 911C as exposuredevices for their respective colors. Each of the optical print heads911K, 911Y, 911M and 911C is the optical print head 800 according to thefourth embodiment.

Image formation sections 910K, 910Y, 910M, 910C include photosensitivedrums 913K, 913Y, 913M, 913C as rotatably supported image carriers andcharging rollers 914K, 914Y, 914M, 914C as charging members foruniformly charging the surfaces of the photosensitive drums 913K, 913Y,913M, 913C. Further, image formation sections 910K, 910Y, 910M, 910Cincludes development units 915K, 915Y, 915M, 915C for forming a tonerimage corresponding to an electrostatic latent image by supplying thetoner to the surfaces of the photosensitive drums 913K, 913Y, 913M, 913Cafter the electrostatic latent image is formed on the surfaces of thephotosensitive drums 913K, 913Y, 913M, 913C by the exposure by theoptical print heads 911K, 911Y, 911M, 911C.

Development units 915K, 915Y, 915M, 915C include toner storage sectionsas developing agent storage sections forming developing agent storagespaces for storing the toner and development rollers 916K, 916Y, 916M,916C as developing agent carriers for supplying the toner to thesurfaces of the photosensitive drums 913K, 913Y, 913M, 913C. Further,development units 915K, 915Y, 915M, 915C include supply rollers 917K,917Y, 917M, 917C for supplying the toner stored in the toner storagesections to the development rollers 916K, 916Y, 916M, 916C anddevelopment blades 918K, 918Y, 918M, 918C as toner regulation membersfor regulating the thickness of a toner layer on the surfaces of thedevelopment rollers 916K, 916Y, 916M, 916C.

The exposure by the optical print heads 911K, 911Y, 911M, 911C isperformed on the uniformly charged surfaces of the photosensitive drums913K, 913Y, 913M, 913C based on image data for the printing. The opticalprint heads 911K, 911Y, 911M, 911C include light-emitting element arraysin which a plurality of light-emitting thyristors as light-emittingelements are arranged in an axis line direction of the photosensitivedrums 913K, 913Y, 913M, 913C.

As shown in FIG. 21, the conveyance section 930 includes a conveyancebelt (i.e., transfer belt) 933 electrostatically attracting andconveying the record medium P, a drive roller 931 rotated by a drivesection and driving the conveyance belt 933, and a tension roller (i.e.,driven roller) 932 forming a pair with the drive roller 931 and applyingtension to the conveyance belt 933.

As shown in FIG. 21, the transfer rollers 940K, 940Y, 940M and 940C arearranged to respectively face the photosensitive drums 913K, 913Y, 913Mand 913C of the image formation sections 910K, 910Y, 910M and 910Cacross the conveyance belt 933. The toner images respectively formed onthe surfaces of the photosensitive drums 913K, 913Y, 913M and 913C ofthe image formation sections 910K, 910Y, 910M and 910C are successivelytransferred by the transfer rollers 940K, 940Y, 940M and 940C to the topsurface of the record medium P conveyed along the medium conveyance pathin the direction of the arrow. Image formation sections 910K, 910Y,910M, 910C include cleaning devices 919K, 919Y, 919M, 919C for removingthe toner remaining on the photosensitive drums 913K, 913Y, 913M, 913Cafter the toner image developed on the photosensitive drums 913K, 913Y,913M, 913C is transferred to the record medium P.

The fixation device 950 includes a pair of rollers 951 and 952 pressedagainst each other. The roller 951 is a roller (namely, heat roller) 951including a built-in heater, while the roller 952 is a pressure rollerpressed against the roller 951. The record medium P with the tonerimages to be fixed passes between the pair of rollers 951 and 952 of thefixation device 950. At the time of passage, the toner images to befixed are heated and pressed and thereby fixed on the record medium P.

A lower surface part of the conveyance belt 933 is provided with acleaning mechanism including a cleaning blade 934, a waste toner storagesection (not shown) and so on.

At the time of printing, a record medium P in the medium cassette 921 isdrawn out by the hopping roller 922 and is sent to the roller pair 923.Subsequently, the record medium P is sent from the roller pair 923 tothe conveyance belt 933 via the registration roller⋅pinch roller 924 andis conveyed to the image formation sections 910K, 910Y, 910M and 910Caccording to the traveling of the conveyance belt 933. In imageformation sections 910K, 910Y, 910M, 910C, the surfaces of thephotosensitive drums 913K, 913Y, 913M, 913C are charged by the chargingrollers 914K, 914Y, 914M, 914C and are exposed by the optical printheads 911K, 911Y, 911M, 911C, and thereby an electrostatic latent imageis formed. The toner formed into a thin layer on the development roller916K, 916Y, 916M, 916C electrostatically adheres to the electrostaticlatent image, and thereby a toner image of each color is formed. Thetoner images of the respective colors are transferred onto the recordmedium P by the transfer rollers 940K, 940Y, 940M and 940C, and therebya color toner image is formed on the record medium P. After the imagetransfer, the toner remaining on the photosensitive drums 913K, 913Y,913M, 913C is removed by the cleaning devices 919K, 919Y, 919M, 919C.The record medium P with the color toner image formed thereon is sent tothe fixation device 950. In the fixation device 950, the color tonerimage is fixed on the record medium P, and thereby a color image isformed. The record medium P with the color image formed thereon isconveyed along the guide 926 and ejected by the ejection roller pair 925to a stacker.

As described above, since the image forming device 900 according to thefifth embodiment employs the optical print head 800 according to thefourth embodiment as each of the optical print heads 911K, 911Y, 911Mand 911C, the print quality of the image forming device 900 can beimproved.

(6) Modification

It is also possible to employ structures obtained by reversing theconductivity types of the semiconductor layers forming thelight-emitting thyristors in the first to third embodiments.

What is claimed is:
 1. A light-emitting thyristor comprising: a firstsemiconductor layer of a first conductivity type; a second semiconductorlayer of a second conductivity type, the second semiconductor layerbeing arranged adjacent to the first semiconductor layer; a thirdsemiconductor layer of the first conductivity type, the thirdsemiconductor layer being arranged adjacent to the second semiconductorlayer; and a fourth semiconductor layer of the second conductivity type,the fourth semiconductor layer being arranged adjacent to the thirdsemiconductor layer, wherein a part of the first semiconductor layer isan active layer adjacent to the second semiconductor layer, a dopantconcentration of the active layer is higher than or equal to a dopantconcentration of the third semiconductor layer, a thickness of the thirdsemiconductor layer is thinner than a thickness of the secondsemiconductor layer, and a dopant concentration of the secondsemiconductor layer is lower than the dopant concentration of the thirdsemiconductor layer.
 2. The light-emitting thyristor according to claim1, wherein a band gap of the active layer is narrower than or equal to aband gap of the second semiconductor layer and narrower than or equal toa band gap of the third semiconductor layer.
 3. The light-emittingthyristor according to claim 1, wherein the third semiconductor layerincludes a gate layer of the first conductivity type and an etching stoplayer.
 4. The light-emitting thyristor according to claim 1, wherein thefirst conductivity type is a P type and the second conductivity type isan N type.
 5. The light-emitting thyristor according to claim 4, whereinthe thickness of the third semiconductor layer is in a range from 150 nmto 180 nm, and the thickness of the second semiconductor layer is in arange from 270 nm to 330 nm.
 6. The light-emitting thyristor accordingto claim 4, wherein a dopant concentration of the fourth semiconductorlayer is in a range from 1.2×10¹⁸ cm⁻³ to 1.8×10¹⁸ cm⁻³, the dopantconcentration of the third semiconductor layer is in a range from 8×10¹⁷cm⁻³ to 1.2×10¹⁸ cm⁻³, the dopant concentration of the secondsemiconductor layer is in a range from 4×10¹⁷ cm⁻³ to 6×10¹⁷ cm⁻³, andthe dopant concentration of the active layer is in a range from 1.2×10¹⁸cm⁻³ to 1.5×10¹⁹ cm⁻³.
 7. The light-emitting thyristor according toclaim 1, wherein the first conductivity type is an N type and the secondconductivity type is a P type.
 8. The light-emitting thyristor accordingto claim 7, wherein the thickness of the third semiconductor layer is ina range from 180 nm to 220 nm, and the thickness of the secondsemiconductor layer is in a range from 270 nm to 330 nm.
 9. Thelight-emitting thyristor according to claim 7, wherein a dopantconcentration of the fourth semiconductor layer is in a range from4×10¹⁸ cm⁻³ to 6×10¹⁸ cm⁻³, the dopant concentration of the thirdsemiconductor layer is in a range from 7×10¹⁷ cm⁻³ to 1×10¹⁸ cm⁻³, thedopant concentration of the second semiconductor layer is in a rangefrom 4×10¹⁷ cm⁻³ to 6×10¹⁷ cm⁻³, and the dopant concentration of theactive layer is in a range from 1×10¹⁸ cm⁻³ to 1.5×10¹⁹ cm⁻³.
 10. Thelight-emitting thyristor according to claim 1, wherein the firstsemiconductor layer includes a first layer, the active layer, and asecond layer arranged between the first layer and the active layer, aband gap of the first layer is wider than a band gap of the activelayer, and a band gap of the second layer is wider than the band gap ofthe first layer and a band gap of the fourth semiconductor layer. 11.The light-emitting thyristor according to claim 10, wherein the secondsemiconductor layer includes a third layer adjacent to the active layerand a fourth layer arranged between the third layer and the thirdsemiconductor layer, and a band gap of the third layer is wider than theband gap of the first layer and the band gap of the fourth semiconductorlayer.
 12. The light-emitting thyristor according to claim 1, furthercomprising: a first electrode electrically connected with the firstsemiconductor layer; a second electrode electrically connected with thesecond semiconductor layer or the third semiconductor layer; and a thirdelectrode electrically connected with the fourth semiconductor layer.13. A light-emitting element chip comprising: a substrate part; and thelight-emitting thyristor according to claim 1 arranged on the substratepart.
 14. The light-emitting element chip according to claim 13, whereinthe first semiconductor layer is arranged on a side closer to thesubstrate part than the fourth semiconductor layer.
 15. Thelight-emitting element chip according to claim 13, wherein the firstsemiconductor layer is arranged on a side farther from the substratepart than the fourth semiconductor layer.
 16. An optical print headcomprising the light-emitting element chip according to claim
 13. 17. Animage forming device comprising the optical print head according toclaim 16.